1. Field of the Invention
The present invention generally relates to a sampling methodology. More particularly, the present invention is directed to a method and apparatus for improving capturing efficiency of airborne particulates and for providing immediate sample treatments which serve to enhance post-collection detection or identification systems.
2. Description of the Related Art
Particulate materials, dispersed in air, pose major threats to the health and safety of the populace. For example, the EPA has strict regulations for respirable dust particles, which may bear allergens capable of triggering severe allergic reactions such as, for example, asthma, in susceptible individuals. Building engineers are plagued with particulate materials, which may include microorganisms, causing sick building syndrome. The FAA and the military have requirements to detect minute particles for identification of explosive materials that may pose a threat to personnel. Finally, the threat of biological warfare requires systems that can efficiently collect and analyze minute quantities of airborne toxins, bacteria, and viruses. All of these critical applications require efficient concentration and collection of airborne particulate materials over a broad particle size range.
For optimum performance, the collection technology must rapidly provide the sample in a form that can be measured by the detector or sensor. For example, many biological sensors operate on a sample that is presented as a solution or suspension in water. For this type of detection device, the collection of sample must be gentle enough to minimize disruption of surface antigens present on the biological agents. On the other hand, sensors based on detecting chemical components of the particles generally require that the particles be disrupted in some manner to free the molecules of interest for detection. For these types of sensors, high-speed impaction onto solid phase surfaces, such as a metal plate coated with mineral oil, provides reasonable collection efficiency and the requisite disruption of the particulate into molecular components. Furthermore, for sensors that employ focused energy, such as a laser, in the detection scheme require that the sample be highly spatially localized on a surface. An example of this technology is laser ionization mass spectrometry. In this technique the particles in the air must be localized in a single sample spot of less than the diameter of the laser beam or ionization region to be effectively analyzed. In one embodiment, this area is required to be a circle with a diameter of the order of about one (1) millimeter.
A variety of sampling devices configured to separate and deliver the material to be tested by a sensor are designed based on how the sample is originated: whether it comes from air, liquid, solid objects, surfaces, or from human tissue. There are several issues that make sampling for biological agents particularly challenging. Firstly, some forms of analysis require living organisms for detection and, therefore, the collection technology must not “harm” the sample. Secondly, the target microbe is generally only one component of a complex matrix of biological elements and chemical compounds that may affect the detection process, so the sample must often be purified to some extent. Finally, the sample must be highly concentrated for a rapid analysis.
Among general types of sampling devices designed to accomplish one or more of these objectives that are of a particular interest within the context of the present invention are viable particle-size impactors and virtual impactors, cyclone samplers and bubblers. Each of these technologies is briefly described below.
A viable particle-size impactors typically has multiple stages. Each stage contains a number of precision-drilled orifices that are appropriate for the size of the particles to be collected in that stage, and orifice sizes decrease with each succeeding impactor state. Particles in the air enter the instrument and are directed towards the collection surface by the jet orifices. Any particle not collected by that stage follows the stream of air around the edge of the collection surface to the next stage. The collection plate is typically a petri dish with agar or other suitable growth medium.
A virtual impactor is similar to a viable particle-size impactor, but uses a collection probe instead of a flat plate as its impaction surface. Air flows through the collection probe and the collected particles are transported to other portions of the collector for additional concentration. By controlling the flow in the impactor, it is possible to adjust the cutoff size to the particles collected. By passing the collection probe airflow into successive virtual impactors, the particles can be concentrated many times the original air concentration before collection.
A cyclone is an inertial device that is commonly used in industrial applications for removing particles from large airflows. A particle-laden air stream enters the cyclone body and forms an outer spiral moving downward towards the bottom of the cyclone. Larger particles are collected on the outer wall due to centrifugal force. Smaller particles follow the airstream that forms the inner spiral and leave the cyclone through the exit tube.
A bubbler or impinger operates by drawing aerosols through a current inlet tube to create a jet. Usually the jet is submerged into the liquid contained in the sampler. As the air passes through the liquid, the aerosol particles are captured by the liquid surface at the base of the jet. In order to collect the smallest particles possible, the jet is typically made with a small critical orifice causing the flow to become sonic. Sampling small particles requires that due consideration be given to the variables that affect aerodynamic characteristics of the particulates, namely the size, number, randomness, and independence thereof. In particular, the size of the captured particulates can radically affect the collection efficiency of the cyclone and virtual impactor devices. The wetted-wall cyclone, and multi-stage virtual impactors feeding into true fluid impactors collect large volumes of air (on the order of about 1000 L/min), concentrate the respirable aerosol particles and impinge these into several milliliters of aqueous collection fluid. With proper tuning, these samplers provide high collection efficiency for particles greater than approximately one (1) micrometer in size. The collection efficiency drops precipitously for particles smaller than one micrometer. The central phenomenon exploited for operation of these devices is impaction of the particles, traveling at the proper velocity, onto a liquid collection surface. Upon the impact, the particles “splash down” into the fluid thereby minimizing particle bounce and reaerosolization into the exhaust airstream. While such samplers provide reasonably gentle capture, the relatively large volumes of collection fluid employed in this system lead to a dilution of the collected sample that makes sample cleanup and detection more difficult. Furthermore, the large volumes of collection fluid, when used for a long period of time, provides high logistics burden for fielded systems.
An alternative strategy, directed to overcome the disadvantages of the above-discussed impactors, is based on collection of sample by impaction onto a polymer tape. Developed at the Applied Physics Laboratory of Johns Hopkins University (JHU/APL), a sampling system, equipped with a polymer tape, features a two-stage virtual impactor particulate concentrator. The latter typically collects air at a rate of 800 L/min and outputs air that is concentrated by a factor of 20×for particles greater than approximately 1 um, at a rate of 15 L/min. The output air enters a five (5) jet true impactor that directs the particles toward the polymer surface where a fraction of them collect on the surface. Generally, however, a large fraction of the particles strike the surface of the tape, bounce off, and are reaerosolized into the airstream. These particles either deposit on an undesirable surface in the sampler or are lost in the exhaust air. The collection efficiency and spot localization for this approach have been found to be low.
To minimize particle bounce and therefore to enhance both the collection efficiency and spot localization, the sampling system designed by JHU/APL has been provided with a polymer tape coated with mineral oil, vacuum grease or an adhesive compound. However, this system requires an oily or sticky coating on the sample tape that tends to deposit on other mechanical motion components of the sampling system resulting in unacceptable performance. Furthermore, the collection efficiency for smaller particles such as viruses, certain types of bacteria and finely dispersed explosives still desires to be higher. Overall, the present technology may have the following deficiencies: (1) relatively low collection efficiency, particularly at small particle sizes; (2) harsh, dry environment in samplers that causes low viability for bacteria and viruses; and (3) poor focusing of deposited material.
Thus, despite the intense activity in the recent past to advance and develop bioaerosol samplers, a need remains for a method and an associated aerosol collector that would enhance particle collection efficiency for current samplers and particularly, for collectors utilizing solid-surface sample tapes. It would also be desirable to provide a method and apparatus characterized by improved identification of the collected particles.